1. Technical Field
The present invention relates to an improved method for a gas phase catalytic oxidation reaction of a hydrocarbon. Specifically, it relates to a method for producing an .alpha., .beta.-unsaturated nitrile (hereinafter referred to as a nitrile) and/or an unsaturated carboxylic acid by subjecting an alkane having a carbon number of from 3 to 8 and/or an alkene having a carbon number of from 2 to 8 to a gas phase catalytic oxidation reaction in the presence of ammonia. Particularly, it relates to an improved method for producing a nitrile using an alkane having a carbon number of from 3 to 8 as a raw material.
2. Background Art
A gas phase catalytic reaction of an alkane having a carbon number of from 3 to 8 and/or an alkene having a carbon number of from 2 to 8, is known as a method for thereby producing an unsaturated carboxylic acid such as acrylic acid, or as a method for producing a nitrile such as acrylonitrile or methacrylonitrile by the reaction in the presence of ammonia. Particularly, acrylonitrile and methacrylonitrile are widely utilized as important raw materials for preparing fibers, synthetic resins, synthetic rubbers, etc., and they are produced on an industrial scale. As a method for producing these nitrites, an ammoxidation method has heretofore been employed as the most common method, wherein an olefin such as propylene or isobutene is subjected to a gas phase catalytic reaction at a high temperature with ammonia and oxygen in the presence of a suitable catalyst.
On the other hand, recently, an interest has been drawn to a method for producing acrylonitrile and methacrylonitrile by an ammoxidation reaction method wherein a lower alkane such as propane or isobutane is used as a starting material instead of an olefin, for an economical reason such as the difference in price between propane and propylene or a difference in price between isobutane and isobutene, and researches and developments of catalysts suitable for such a reaction, have been carried out and various reports have been made. As examples of such reports, a Mo--Bi--P--O type catalyst (JP-A-48-16887), a V--Sb--O type catalyst (JP-A-47-33783, JP-B-50-23016), a Sb--Sn--O type catalyst (JP-B-47-14371), a Sb--Sn--O type catalyst (JP-B-50-28940), a V--Sb--W--P--O type catalyst (JP-A-2-95439), a catalyst obtained by mechanically mixing a V--Sb--W--O type oxide and a Bi--Ce--Mo--W--O type oxide (JP-A-64-38051), a Cr--Sb--W--O type catalyst (JP-A-7-157461) and a Mo--Sb--W--O type catalyst (JP-A-7-157462) are, for example, known. Further, the present inventors have also reported on e.g. a Mo--V--Nb--Te--O type catalyst particularly suitable for this method (JP-A-2-257, JP-A-5-208136).
However, in each of the methods using these catalysts, the selectivity for intended nitrites is not necessarily adequate, and they are not yet satisfactory as industrial methods. Further, in order to improve the selectivity for nitrites, a method has, for example, been proposed wherein a small amount of an organic halide, an inorganic halide or a sulfur compound, is added to the reaction system, but such a method has a problem such as corrosion of the reaction apparatus or complication in the purification of the formed nitrites, and each method has a practical difficulty in industrial application.
The present inventors have studied the characteristics of this reaction in detail in order to improve the selectivity from an alkane to the desired nitrile and as a result, have found it possible to form a nitrile at high selectivity by adjusting the ratio in concentration of alkane:ammonia:oxygen in the reaction gas to be supplied, within a predetermined range, and by suppressing the conversion of the supplied alkane to a level of at most a predetermined value, and they have further found it possible to substantially increase the yield of the nitrile from the alkane by separating the nitrile in an efflux from the reactor and supplying a recovered gas containing an unreacted alkane again into the reactor, and have previously proposed (JP-8-255338).
Further, the present inventors have studied a method for producing nitrites efficiently by one of the above-mentioned methods, or by a combination of a plurality of such methods, or by replacing such methods, and they have found it possible to effectively control the reaction temperature and the quantity of heat generated by the reaction, by the presence in the reactor of a predetermined amount of particles inert to the reaction other than the catalyst particles, whereby nitrites can more effectively be produced, and thus have arrived at the present invention.
Further, the present invention has been accomplished on the basis of a discovery that this method provides the same effects also in a gas phase catalytic oxidation reaction in a fluidized bed reactor of not only an alkane having a carbon number of from 3 to 8 but also an alkene having a carbon number of from 2 to 8.
Further, in the present invention, it is proposed to withdraw the mixed metal oxide catalyst used for the reaction in the form of a mixture with the particles inert to the reaction and to separate and recover the inert particles from the mixture, and an extensive study has been carried out on a method for such separation and recovery, whereby it has been found it possible to accomplish the above object by permitting the inert particles to be present again in the reactor.